In ultrasonic cleaning, a transducer, usually piezoelectric but sometimes magnetostrictive, is secured to a cleaning tank to controllably impart ultrasonic vibrations to the tank. The tank is filled with a cleaning liquid and parts are immersed into the liquid to be cleaned by ultrasonic agitation and cavitation. Interaction between the ultrasonically agitated liquid and the contaminants carried by parts immersed in the liquid causes the contaminants to be dislodged.
Various circuits have been configured for driving the ultrasonic transducer and have provided a variety of features. Parameters which are available for adjustment or control are the ultrasonic frequency, the power level, amplitude or frequency modulation, and duty cycle control of power bursts, among others.
In ultrasonic cleaning, it is known that the output circuit, which usually includes a driver, the ultrasonic transducer, and the mechanical system which it drives have a resonant frequency. The mechanical system, of course, includes the liquid in the tank and the parts immersed in the liquid. Quite clearly, the mass and shape of the parts, the temperature of the liquid, and other factors, all influence the resonant frequency of the output circuit. In some cases, the driver circuit is tuned to the approximate resonant frequency of the load by selection of inductance and capacitance values, and in other cases, the driver can drive the load either on or off resonance.
The art has developed to allow users various controls over the frequency of the ultrasonic generator in an effort to match the resonant frequency of the generator to that of the mechanical system. Indeed, it has been proposed to allow the system to automatically tune to the resonant frequency of the system, but such automatic resonance tuning has not been entirely satisfactory, particularly when combined with other forms of ultrasonic generator control. In Ratcliff U.S. Pat. No. 4,554,477, for example, the output circuit which includes the driver and the load is tuned for automatic resonance tracking, but the power output is intentionally varied or modulated to produce peak power which is substantially higher than the average power output.
It has been proposed to allow the user to adjust a sweep frequency, i.e., a cyclical change in output frequency with respect to time. The aforementioned Ratcliff patent includes a sweep feature to allow sequential resonating of a number of ultrasonic transducers driven in series. Other controls which have been proposed include duty cycle control of the output frequency (bursts of ultrasonic pulses with duty cycle controlled on and off intervals), amplitude modulation of the ultrasonic pulses, and the like. Various forms of power controls have also been proposed. Krsna U.S. Pat. No. 4,864,547 exemplifies a typical approach of using a shunt resistor in one of the main power supply circuits as a general indicator of power delivered to the load. But the measure is indirect and inaccurate because it relates primarily to input power and does not take account of output efficiency.
A further example of an ultrasonic generator including multiple controlled parameters can be found in William Puskas, U.S. Pat. No. 4,736,130. That patent discusses adjustments for the center frequency of the ultrasonic drive, the on and off time of power bursts of the ultrasonic pulses, degas on and off time, as well as amplitude modulation of the ultrasonic power bursts.
It is understood that there is a relationship between the output frequency of the generator, its relationship to the resonant frequency of the system, and the power delivered to the output circuit. Various systems have attempted to monitor power by, in effect, measuring input power to the generator. Thus, ultrasonic controls have been available which claim to be constant power, but which simply include a shunt resistor in the input power circuit which is, at best, a crude indicator of output power, since efficiency and the like are dependent upon the degree to which the system is on or off resonance. It is known, for example, that as the frequency varies from a resonance point to an off resonance point the efficiency of the system decreases, and the power delivered to the load is also reduced.
Thus, it is not a simple matter to propose a multiply controlled ultrasonic generator which is capable of producing constant power, because variation of the frequency parameters, for example, has a direct impact upon the power delivered to the output. Nor is it possible to simply measure input current to the system, and use that as a basis for suggesting constant output power, particularly in systems which allow the variation of center frequency of the ultrasonic drive or its sweep.
Thus, while it has been thought desirable to provide the user with features such as automatic resonance seeking, constant and adjustable power output, and the like, it has not been possible heretofore to provide those features in the same ultrasonic generator. Tradeoffs were necessary due at least in part to the state of the art.